Small angle x-ray scattering studies will be performed on an endonuclease protein (FEN-1). This protein is important in DNA replication and damage repair. It recognizes a specific type of branch defect in the DNA which it specifically repairs. DNA binding and repair seem to be separate properties of this protein. We also have mutants which can still bind DNA but have lost their repair ability. Also, repair requires magnesium to be present. We intend to measure 4 samples, the wild type (+/- Mg) and the repair inactivated mutant (+/- Mg). A high-flux synchrotron beam and an adequate low-background SAXS facility are necessary to get enough data statistics from these dilute solution samples (approx. 1.5 mg/ml).